Polymeric coated metal strip and method for producing same

ABSTRACT

The present invention is a process for continuous extrusion coating the surfaces of a metal strip which includes the steps of moving said metal strip by means of a plurality of rolls. The strip then has impinged on its first and second surfaces to be coated an oxidizing flame. The first surface passes through a corona discharge and extruded on to the first surface is a melted polymeric extrudiate. Thereafter, the surface is passed over a chill roll having a temperature less than said extrudiate to provide a first coated surface. The second surface is then subjected to a oxidizing flame and corona discharge and a melted polymeric extrudiate is extruded on to the second surface. Thereafter, the second surface is passed over a chill roll having a temperature less than said extrudiate to provide a second coated surface. The coated strip is then reheated to about the melt temperature of said extrudiates and quenched to provide an extrusion coated strip of the present invention.

CROSS REFERENCE

[0001] The present application is a continuation-in-part of U.S. application Ser. No. 09/490,305, filed on Jan. 24, 2000 entitled Polymeric Coated Metal Strip and Method for Producing Same.

FIELD OF THE INVENTION

[0002] The present invention provides a novel polymeric coated strip metal and a method for producing same and in particular to a method in which the polymeric coating is extruded onto at least one side of a moving strip of metal, preferably steel.

BACKGROUND OF THE INVENTION

[0003] It is generally well known that for many applications it is desirable to coat a metal strip before is formed into its final shape. Various coatings have been used for decoration or protection of the metal in its intended application. Metal coating such as tin, zinc and aluminum have been used for many years and have been produced using a number of well known methods. In recent years the use of synthetic coatings such as alkyd, vinyl, phenolic, and epoxy resins have been used to coat steel strip. However, many of these coating systems are not useful in continuous strip coating where coils of strip are coated in a single continuous process from a coil uncoiler to coiler.

[0004] A number of processes have been proposed for laminating metal sheet. For example, in U.S. Pat. No. 5,149,389, a laminate metal sheet is described having a noncrystalline polyester film adhered to at least one of its major surfaces. In that case, a polyethylene terephthalate polyester material is use to form a film which is laminated to a metal sheet heated to a temperature above the polyester film such that the outer surface of the film remains above the melting point of the film during lamination and, thereafter, reheating the laminate to a temperature above the melting point of the film for a period of time and then rapidly quenching the polyester coated metal to below the glass transition point of the resin.

[0005] In U.S. Pat. No. 5,985,080 process is described for producing a resin metal laminate. In this process a heating zone for the metal substrate is provided together with a downstream die for feeding a thermoplastic resin in film form onto the metal substrate. Temperate laminate rolls are used for adhering the thermoplastic resin to at least one of the surfaces of the substrate. It is stated that the benefit of the process taught therein is that the metal resin laminate is useful for the manufacture of cans which requires a high degree of processing such as deep draw formation, bend elongation and ironing working. In EPO patent application 0,067,060 there is provided a method of applying a thermoplastic resin directly to a plate without first forming a film for lamination.

[0006] More recently, in U.S. Pat. No. 5,919,517, there is taught a method for continuously coating metal strip, especially aluminum, without first forming a film. The process comprises moving a heated strip between a pair of rolls and extruding onto the heated strip a polymeric coating and drawing the extruded polymeric coating to reduce its thickness. After such drawing the coated strip is fed through a pair of rolls to adhere the coating to the strip and, thereafter, repeating the process for the other side of the strip.

[0007] While many of the prior art methods provide coated metal strip or sheet, many of them require the formation of a film for lamination. On the other hand, some methods which provide direct coatings on to the metal substrate do not provide a continuous process for strip which operates from uncoiler to coiler or does not provide a strongly adherent coating so a to permit deep drawing or metal forming without delamination.

[0008] Accordingly, it is an object of the present invention to provide a method for the continuous coating of metal strip, particularly various types of steel strip, with a thermoplastic resin coating. It is a further object of the present invention to provide a method of coating a metal strip from uncoiler to coiler at relatively high speed with uniformity of coating thickness and consistency of texture and color. Finally, it is an object of the present invention to provide metal composite comprising metal strip having a tightly adhered uniform polymeric coating. Other advantages of the invention will be apparent from a description of the invention.

SUMMARY OF THE INVENTION

[0009] The present invention comprises a novel coated strip composite and method for making same. Generally, the method includes the steps of pretreating a continuous metal strip with an oxidizing flame treater. In one embodiment of the invention, the flame pretreatment is juxtiposed with the strip uncoiler. As a practical matter, placing the flame pretreatment adjacent the uncoiler may have the advantage of reducing the space required for the coating process rolls and extruders. In another embodiment of the invention, the strip is pretreated with the Corona Discharge first and then flame pretreated.

[0010] After flame pretreatment, the strip passes through a corona discharge unit. Depending whether upon whether one or two sides of the strip is to be coated or the coating extrusion is done substantially simultaneously, the corona unit is placed ahead of the respective extruder. In a presently preferred embodiment of the invention, a polymeric coating is applied to the two sides in two steps. In the first step, the surface to be coated is subjected to an oxidizing flame treatment, but the second side is subjected only to the corona discharge. However, in another embodiment where the coating is applied in two steps, both sides of the strip are subjected to an oxidizing flame and a corona discharge prior to the extrusion coatings being applied.

[0011] The coatings are applied by a polymeric extruder and die located just after a nip roll on the processing line. The extruder can be a single or double screw as is well known in the art. The extrusion temperature for polypropylene pellets, for example is 575 to 585° F., but as clear to those skilled in the art, the temperature will vary depending upon the resin used for the coating and coloration pigments admixed to provide a colored coating. Immediately following the extruders is a chill roll preferably at a temperature substantially less than the extrudate temperature. For example, in certain high speed operations., e.g., 500 to 1500 feet per minute, temperatures of from about 150 to 170° F. are preferred for polypropylene.

[0012] In a two stage coating process another set of rolls and an extruder is positioned to apply an extrudate on to the uncoated side. In one embodiment, it possible to position the extruders substantially adjacent to each other to provide coatings on both sides of the strip substantially simultaneously. In this latter embodiment, it is preferred to have a pair of chill rolls to remove heat from each coated surface prior to entry into a remelt/reheat oven. In any event, after the second surface is coated the strip is fed through a reheater, for example an induction oven, to cause the coating to bond to the substrate and provide an enhance surface finish on the coating. Typically a temperature of 285° F. to about 500° F. has been found beneficial depending upon the degree of crystallization desired and the type of polymers employed.

[0013] After the reheat treatment, approximately 1 to 15 seconds, the strip is passed through a quench tank comprising fluid maintained at a temperature of 40° to 120° F. The temperature depends upon the polymer. The initial quench fluid contact with the coated strip is maintained in laminar flow condition. Thereafter the strip can be dried and gauged prior to recoiling or subsequent processing or application forming or drawing. Other advantages of the processes of the present invention will become apparent from a perusal of the following detailed description of presently preferred embodiments taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a diagrammatic view of a process coating line in accordance with the present invention;

[0015]FIG. 2 is an exploded diagrammatic view of a typical extruder used in lines shown in FIGS. 1 and 2; and

[0016]FIG. 3 is a diagrammatic view of another embodiment of a coating line in accordance with the present invention.

PRESENTLY PREFERRED EMBODIMENTS

[0017] Referring to FIG. 1, the present invention comprises coating processing line 10 having an uncoiler 12 for uncoiling a strip 11 of metal, such as steel, tinplate, chromium-coated steel, electrogalvanized, etc. While shown in FIGS. 1 and 3, various steering and bridle rolls will not be described or discussed in detail. Such rolls are placed where necessary and well known by those skilled in the art and form no part of the invention described herein.

[0018] A flame treater 13, for example, a Flynn burner, is positioned downstream of uncoiler 12. In the preferred embodiments of the invention, it has been found desirable to orient the burner heads between about 25° to 155° but preferably at 90° to the moving strip. It is then highly preferable to position the cones of the flame themselves to just make contact with the moving strip and to adjust the air/fuel mixture to achieve an oxidizing flame with a plasma value between 30 and 50 but preferably between 37 and 42 at strip speed up to 500 ft/min. and much higher values at higher strip speeds. Flame treatment can be applied to either non-coated side of the strip but preferably to the side of the strip to be extrusion coated.

[0019] Following the flame treater 13, strip 11 then enters a Corona discharge, for example, an Enercon Corona, which further pre-treats the side of the strip to be extrusion coated with from 5 to 30 watts/ft²/min. and preferably about 6 watts/ft²/min.

[0020] In this embodiment, strip 11 is entrained over rollers to position the strip so that one of its surfaces is juxtaposed to a first extruder 15 having die 16 for extruding the melted polymer on to the strip surface. The strip is typically at ambient temperature or slightly above that when it enters the extruder die area; preheating is not required. Die 16 is positioned so that the melt curtain exiting the die drops vertically into the nip area between the moving strip 11 and chill roll 18 such that the melt film contacts the moving strip just prior to contacting the chill roll. The distance of the die lip to the strip/chill roll nip is between 3 and 10 inches but preferably between 4 and 6 inches although the minimum distance achievable will be dictated by the dimensions of the nip roll and chill roll employed. Nip roll 17 defines the strip/chill roll nip area and functions to apply sufficient pressure to the moving strip (and thus to the chill roll through the melt film) to effect intimate contact of the melt film with the moving strip. This nip roll can be covered with any number of durable rubber coatings but preferably neoprene with a hardness between 80 and 90 durometer but preferably 85-90. The nip roll may also be chrome coated steel roll.

[0021] One or more extruders may be utilized to apply one or more layers of polymer coating in sequence or simultaneously. Two hoppers, implying two extruders per extruder/die station, are shown in FIG. 2. After exiting chill roll 18, the moving strip 11 proceeds around stripper roll 20 which is designed to remove the coated strip from the chill roll after the melted coating is initially cooled by chill roll 18. The temperature of chill roll 18 is maintained between 100° and about 190° F. but preferably between 140° to 160° F. for a polymer leaving die 16 at a temperature of about 540° to 590° F.

[0022] Upon leaving stripper roll 20, strip 11, coated on the first side passes through an edge trimming operation and then is entrained over rolls to position the second uncoated side for treatment with a second flame treater 21A and a second Corona discharge unit 21 and finally over nip roll 26 to a position adjacent to second extrusion station 25 and 26. As in the previous case, flame treater 21A is positioned preferably at 90° to the moving strip with an air/fuel mixture to achieve a preferable plasma value of from 37 to 42 and further pretreatment with Corona discharge unit 21 is accomplished at a preferred watt density of 6 watts/ft²/min. After passing through Corona discharge unit 21, strip 11 enters extruder 25 and die 26 area where the second surface has applied thereto a polymeric coating. It may be desirable depending upon line length, speed of line, or other parameters, to provide the use of ovens to maintain the temperature of the first coated surface.

[0023] As shown, extruders 15 and 25 can be single or twin screw type extruders and can have additional hoppers and material feed systems for providing such additional components as a pigment concentrate for coloration of the coating, or polymers or copolymers either alone or as carriers for other components. A number of different polymer resins have been used such as polypropylene, polyethylene, PET and the like. In addition, various pigment concentrates, such as polypropylene-based TiO₂, and others have been used to provide coloration to the coating. Other functional additives, such as stabilizers, lubricants, nucleators, clarifiers, etc., have also been added to provide various chemical and physical property attributes.

[0024] After exiting chill roll 28 and stripper roll 27, strip 11 which is now coated on both sides with extrudate coating passes through an edge trimming operation and then through remelt oven 32 which is preferably an induction oven used to remelt the polymer coating to a temperature of from 265° to 650° F. but preferably between 470° to 490° F. for polypropylene. At these conditions, the polymeric or copolymeric coating tends to flow which provides somewhat of a leveling effect which enhances the surface finish through filling of voids and depressions in the original coating. It is also this treatment which is responsible for development of final adhesion characteristics of the coating to the strip. At this point, pressure rolls can also be used to adjust the coating thickness and/or surface finish or enhance the adhesion of the coating prior to quenching. For example; the hot strip can pass through gauge rollers or strippers 36.

[0025] The reheated strip 11 is then directed to quench facility 34 in which a fluid such as water or other compatible heat transfer fluid is used to cool the polymer/strip combination. It is preferable for the heat transfer fluid to be directed to provide a laminar flow with respect to the strip moving therethrough in order to provide an enhanced polymer appearance and adhesion quality. It is desirable to provide a slight delay between the induction oven 32 and quench 34 of between 1 and 15 seconds. However, it has been found to be adequate to use a delay of 1 to 5 seconds between reheating and quenching. The temperature of the quench liquid is preferably about 35° to 120° F., preferably 60 to 80 F. The coated strip 11 may then pass through a surface treatment station where the polymer surface is corona treated to allow for other finishing operations such as painting, decorating or lamination. The coated strip 11 is then recoiled onto coiler 38.

[0026] The following examples are illustrative of the presently preferred embodiments of the invention.

EXAMPLES Example #1

[0027] Coating trials were completed on line 10 described with respect to FIG. 1 to produce two layer coated steel substrates Malaic anhydride enhanced, high melt strength polypropylene homopolymer was used as a tie layer. High melt strength polypropylene homopolymer, both with and without TiO₂ concentrate as a colorant, was used as a bulk layer. A ratio of ˜20:80 tie layer to bulk layer was used. Metal substrates consisted of 75 pound per base box Tin Free Steel TU Designation and 65 pound per base box Tin Free Steel DR-9 Designation. Flame and Corona discharge were used in tandem as metal strip pretreatments to clean and prepare the strip surfaces to accept the coating and to provide sufficient green strength adhesion of the applied coating for transport to the post-treatment section. Post-treatment consisted of heating the initially coated metal strip to a temperature of 485° F. followed by a 5 second delay prior to water quench using a laminar flow arrangement. Resulting coated products were subjected to various laboratory tests to determine chemical and physical properties as well as formability testing which resulted in the production of 307 sanitary ends and 110×307 shallow draw cans.

Example #2

[0028] Coating trials were completed as described in Example #1 except the bulk layer consisted of random copolymer (RACO) and mixtures of RACO and polypropylene homopolymer, both with and without TiO₂ concentrate. Substrates consisted of standard 75 pound per base box tin Free Steel TU Designation designated for the production of sanitary ends and 75 pound per base box tin Free Steel TU Designation (Special Hot Mill Practice) Tin Free Steel designated for use in the production of 0.5 and 1.0 pound tapered salmon cans. Ends and cans produced from these coated products were evaluated using enamel rater measurements and standard test pack analysis to determine coating integrity and performance with various food products.

Example #3

[0029] Coating trials were completed as described in Example #1 except the bulk layer consisted of polypropylene homopolymer, random copolymer, medium impact copolymer, or a combination of these three plus one of several additives consisting of various clarifiers and nucleators to modify crystallization mechanisms for the coatings on food cans and sanitary ends subjected to processing (i.e., retort cooking) after packing. Laboratory tests performed on shallow draw cans and sanitary ends produced from these coated materials consisted of retort @ 250° F. & 15 psi for 90 minutes followed by characterization in critical, stressed areas of the cans and ends using binocular microscopy as well as scanning electron microscopy (SEM).

Example #4

[0030] Coating trials were completed as described in Example #1 except differential coatings, consisting polypropylene homopolymer on one side and polypropylene homopolymer plus TiO₂ concentrate on the other side, polypropylene homopolymer on one side and polypropylene homopolymer plus a commercial gold concentrate on the other side, and polypropylene homopolymer plus TiO₂ concentrate on one side and polypropylene homopolymer plus commercial gold concentrate on the other side. Substrates consisting of 75 pound per base box Tin Free Steel TU Designation and 75 pound per base box Tin Free Steel TU Designation (Special Hot Mill Practice) Tin Free Steels were once again employed using flame and Corona discharge pretreatments in tandem and post-treatment of the applied coating at various temperature ranging from 450° to 485° F. depending on preestablished calibration for each specific coating with the emissivity measurement device.

Example #5

[0031] Coating trials were completed as described in Example #1 except substrates utilized consisted of 65 pound per base box Tin Free Steel DR-7.5 Designation, 0.20 lb/Base Box Exlectrolytic Tinplate Coating and 65 pound per base box DR-8 Designation, 0.20 lb/Base Box Eltrolytic Tinplate Coating. Pretreatment consisted of the use a flame, only, with no Corona discharge. Initially coated products were post-treated at temperatures between 440° and 480° F. and resulting post-treated materials were evaluated using standard adhesion testing and test pack analysis. While presently preferred embodiments have been shown and described in particularity, the invention may be otherwise embodied within the scope of the appended claims.

Example #6

[0032] Coating trials were completed as described in Example #1 except 65 pound per base box Full Finished Blackplate was used as the substrate with applied coating consisting of those as defined in Example #4. Excellent coating adhesion characteristics were observed for this coated product when post-treated at 485° F. with a delay time of 5 seconds prior to laminar flow water quench.

[0033] While presently preferred embodiments of the invention have been shown and described in particularity, the invention may be otherwise embodied within the scope of the appended claims. 

What is claimed is:
 1. A process for continuous extrusion coating one of the surfaces of a metal strip comprising the steps of: a) moving said strip by means of a plurality of rolls; b) impinging at least one of said surfaces to be coated with an oxidizing flame only or an oxidizing flame followed by a Corona discharge; c) extruding on to said impinged surface of said moving strip a melted polymer extrudate and then passing said surface over a chill roll having a temperature less than said extrudate to provide a first coated surface; d) reheating said moving strip having a first coated surface to about the melting temperature of said extrudate; and e) quenching said reheated strip.
 2. A process for continuous extrusion coating both surfaces of metal strip comprising the steps of: a) moving said metal strip by means of a plurality of rolls; b) impinging at least one of said surfaces to be coated with an oxidizing flame only or an oxidizing flame followed by a Corona discharge; c) extruding on to said impinged surface of said moving strip a melted polymer extrudate and therefore passing said surface over a chill roll having a temperature less than said extrudate to provide a first coated surface; d) subjecting said non-coated surface to a second oxidizing flame treatment only, a Corona discharge treatment only, or an oxidizing flame treatment followed by a Corona discharge; e) extruding onto said non-coated surface of said moving strip a melted polymer extrudate and therefore passing said surface over a second chill roll having a temperature less than said extrudate to provide a second coated surface; f) reheating said moving strip having a first and second coated surface to about the melt temperature of said extrudate; and g) quenching said reheated strip.
 3. A process for continuous extrusion coating the surfaces of a metal strip comprising the steps of: a) moving said metal strip by means of a plurality of rolls; b) impinging on the first and second surfaces to be coated an oxidizing flame; c) passing a first surface through a Corona discharge; d) extruding on to a first surface a melted polymer extrudate and therefore passing said surface over a chill roll having a temperature less than said extrudate to provide a first coated surface; e) subjecting said second surface to an oxidizing flame treatment only, a Corona discharge treatment only, or an oxidizing flame followed by a Corona discharge; f) extruding on to said second surface a melted polymer extrudate and therefore passing said surface over a second chill roll having a temperature less than said extrudate to provide a second coated surface; g) reheating said moving strip having a first and second coated surface to about the melt temperature of said extrudates; and h) quenching said reheated strip to provide an extrusion coated strip.
 4. A process as set forth in claim 1 , 2 , or 3 wherein said moving strip is between about 150 ft/min. to 1500 ft/min.
 5. A process as set forth in claim 1 , 2 , or 3 wherein the polymer is heated to a temperature necessary for extrusion coating and said chill rolls are maintained between about 100° F. and 190° F.
 6. A process as set forth in claim 1 , 2 , or 3 wherein the melted polymer film exiting the die lip is positioned from 24″ and 6″ above the moving strip/chill roll interface with the melt being applied to the moving strip at a position just prior to contact of the coated metal surface with the chill roll surface.
 7. A process as set forth in claim 1 , 2 , or 3 wherein the nip roll consists of neoprene or steel applied by pressure against a chill roll.
 8. A process as set forth in claim 1 , 2 , or 3 wherein the moving strip is initially pre-treated using a flame with a plasma value between 30 and 50 plasma units and positioned at approximately 90° to the moving metal strip such that the cones of the flame just touch the strip surface.
 9. A process as set forth in claim 1 , 2 , or 3 wherein initial flame pre-treatment is followed by a second pre-treatment consisting of a Corona discharge operated in a range between 5 to about 30 watts/ft²/min.
 10. A process as set forth in claim 1 , 2 , or 3 wherein the coated metal strip exiting the chill roll/striper roll is post-treated by re-melting the coating in by heating the strip to a temperature between 265° F. to about 550° F., a delay before quench of from 1 second to 5 seconds, and a laminar flow water quench at a temperature ranging from 35° F. to about 120° F.
 11. A process as set forth in claim 1 , 2 , or 3 wherein the metal strip to be coated is selected from the group consisting of electrolytic tinplate, electro-coated chromium steel, electro-coated galvanize steel, hot-dip galvanized steel (HDG), blackplate, cold rolled steel strip, aluminum coated steel, all types of stainless steels, hot rolled strip, Galvalume coated steel and nonferrous metal strip.
 12. A process as set forth in claim 1 , 2 or 3 wherein said melted polymer of coating extrudate or co-extrudate of at least one polymer selected from the group comprises at least one layer and adhesion consisting of polypropylene, copolymers of polypropylene, polyethylene, copolymers of polyethylene terephthalate, copolymers, polybutylene terephthalate blends of polypropylene, polyethylene, polybutylene homopolymer polyethylene and polybutylene random copolymer, nylon, copolymers of nylon and optionally an adhesive admixed with such selected polymer.
 13. A process as set forth in claim 12 wherein said layer includes one or more colorant pigments, nucleators, antimicrobial additive, and friction modifier.
 14. A polymer coated metal strip having an extruded or co-extruded polymeric coating with a total thickness from about 0.3 to 15.0 mils, a high modulus of bending, and can be drawn or ironed wherein the adhesion of said coating is sufficiently good such that the coating does not delaminate during such operations.
 15. A polymer coated metal strip as set forth in claim 13 wherein the metal strip is as set forth on claim 11 and the polymer coating consists of at least one extruded polymer layer.
 16. A polymer coated metal strip as set forth in claim 13 wherein a first layer comprises a blend of polypropylene homopolymer with a grafted maleic anhydride polypropylene component.
 17. A polymer coated metal strip as set forth in claim 13 wherein a bulk layer is selected from at least one of the group and consists of primarily polypropylene homopolymer, polypropylene random copolymer, a blend of polypropylene homopolymer and polypropylene random copolymer, polyamides, EVOH, and polyethylene terephthalate.
 18. A polymer coated metal strip as set forth in claim 13 wherein at top layer consists of polypropylene homopolymer and polypropylene random copolymer combined with polymeric components having at least one chemical and physical property to provide hardness, scuff/scratch resistance, brittleness, stain resistance, or UV resistance to said top layer.
 19. A process as set forth in claim 1 , 2 , or 3 wherein the coated metal has the polymer surface of said strip treated, for instance by corona discharge to enhance post decoration quality.
 20. A process as set forth in claim 1 , 2 or 3 wherein said extruded film is an extrudable material or a blend of extrudable materials selected from the group consisting of polypropylene, polybutylene terephthalate, polyethylene terephthalate, polyethylene, polyester, nylon (polyamide-PA), ethlene copolymers, such as ethylene vinyl acetate and ethylene n-butyl acrylate, vinyl acetate, normal butyl acrylate and polyvinylidene chloride 